Method for sterilizing liquid food products

ABSTRACT

A method and apparatus for sterilization of liquid food products utilizing a generally cylindrical pressure vessel. The liquid product to be treated is introduced at the top of the vessel in such a manner as to form an insulating barrier of liquid product between the heated interior and any interior surfaces of vessel which the product comes in contact with including the vessel side walls. Steam is introduced through a centrally located diffuser intermediate the top and bottom of the vessel for heating the liquid to a sterilization temperature. A gas removal pipe is provided for removing air and gases released from the liquid product and remaining after condensation of the steam from the lower portion of the vessel. The insulating barrier of liquid product may be formed by the use of a plurality of spray nozzles, by a combination of spray and direct film formation, or by direct film formation alone.

This is a division of application Ser. No. 872,135 filed 06/09/86 andentitled "Method and Apparatus for Sterilization of Liquids", now U.S.Pat. No. 4,776,268.

BACKGROUND OF THE INVENTION Field Of The Invention

The present invention relates to an improved method and apparatus forsterilizing liquid food products through the use of steam infusion.

BACKGROUND INFORMATION Description Of The Prior Art

It has long been recognized that certain food products lend themselvesto preservation by the use of heat to sterilize the product.Sterilization destroys bacteria and inactivates most enzymes which causeproduct spoilage. Additionally sterilization of certain food productsmay eliminate the need for refrigeration. This is particularly true inthe case of dairy products such as milk or ice cream mix, to which thisinvention is primarily directed, although it may be equally applied toother liquid products such as juices.

In the sterilization of milk, it is necessary to raise the temperatureof the milk sufficiently to destroy all bacteria and inactivate enzymeswhich cause spoilage. The rate of destruction or inactivation of theseorganisms varies with both temperature and the time during which theproduct is held at an elevated temperature. A preferred method ofsterilizing milk and dairy products has been to utilize steam infusionto subject the milk to ultra high temperatures for very short periods oftime followed by flash cooling. This has been proven to achieve superiorproduct flavor. Various approaches have been used in the past toaccomplish this. For example U.S. Pat. No. 3,156,176 to Wakemandescribes a heating apparatus in which steam is supplied into a chamberwith the liquid product being introduced in the form of a curtain-likefilm to expose the fluent product to the elevated steam temperatures.Similarly, U.S. Pat. No. 2,899,320 to Davies and U.S. Pat. No. 3,032,423to Evans, both utilize apparatus for containing steam in which theproduct is passed over plates within the steam chamber and heated whilethe product flows downwardly to a collection point for delivery to aflash chamber. A variation of this method is also described in U.S. Pat.No. 3,771,434 to Davies in which screen panels are used to form a thinfilm of product for exposure to steam. One major disadvantage of themethods and apparatus described in the foregoing patents is the factthat liquid food products, particularly milk products, have a tendencyto burn and collect on heated surfaces which are at temperatures greaterthan or equal to the temperature of the product itself. Such burning, inaddition to fouling the apparatus itself necessitating periodiccleaning, also results in undesirable flavor changes to the milkproduct.

In an obvious effort to avoid such burn-on and fouling, U.S. Pat. No.4,310,476 to Nahra and U.S. Pat. No. 4,375,185 to Mencacci attempt toform free falling thin films of milk within a steam atmosphere forraising the product temperature. A problem associated with attempting toform a free falling thin film is that the integrity of such films isvery unstable and are subject to splashing or break-up in the presenceof moving or circulating steam and gases. Film formation requires closeadherence to flow parameters and such devices are also subject to theproduct burn-on problems when hot surfaces are contacted. Additionally,it is recognized as discussed in the Nahra patent that physicalagitation of milk may also affect the ultimate flavor of the treatedproduct and disturbance of the free falling films will result in suchagitation.

Another problem associated with many of the prior art approaches tosteam infusion of liquid products is that the devices are not easilycleaned for example with the use of clean-in-place systems. The moreinternal components in which the product may collect or burn-on, themore difficult the cleaning process.

A further problem which has been recognized in the steam infusionprocesses is dealing with the presence of entrained gases released fromthe product when heated and gases, air or oxygen remaining after thesteam condenses into the product. As noted by A. G. Perkin, "Review ofUHT Processing Methods" journal of the Society of Dairy Technology, Vol.38, No. 3, July, 1985, gases trapped within the pressure vessel resultin increased pressures and greater temperature differences between thesteam and product which can again increase the risk of overheating theproduct and undesirable flavor changes.

Finally, in an effort to reduce burn-on, U.S. Pat. No. 3,771,434 toDavies and the patent to Nahra both suggest the use of cooling mediumsin the lower levels of the chambers where the product is collected. Theuse of cooling jackets, however, reduces the efficiency of the steamchamber by increasing the internal operating pressure and amount ofcondensation of steam thereby requiring additional quantities of highertemperature steam to make up for such losses. This directly effects theefficiency of the process as well as the flavor of the end product.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus forsterilizing liquid food products with pressurized steam, in which all orat least a portion of the product to be sterilized is supported andguided to form a continuous flowing film over the entire interiorsurface of a pressure vessel so that the liquid product itself acts asan insulating barrier between the steam and vessel side wall and noportion of the product is ever in contact with any internal surfacehotter or equal to its own temperature.

A further object of the invention is to provide for central introductionof saturated steam into a pressurized vessel to heat a surroundingcontinuous film of liquid and/or sprayed liquid to be treated andsterilized.

A further object of the invention is to provide for the removal ofreleased air and other gases from the vessel without removal of steam.

The present invention comprises a method and apparatus for sterilizingliquid food products in which the food products are introduced at thetop of a pressurized vessel and directed to form a continuous film ofliquid which also may be sprayed so as to flow and fall downward alongthe internal surfaces of the vessel to a collection point. The productitself is used to form a continuous film directly at the top of thevessel along its upper side walls. Either direct flow or spray nozzlesmay be used to form a continuous insulating flowing film onto theinterior vessel side walls, or a combination of spray and direct flowmay be employed. Saturated steam is gently diffused centrally through aporous cylinder, to elevate the temperature of the fluid spray andflowing down the internal side walls of the vessel, to heat the liquidfilm and spray of product by infusion. A concentrically located centraltube is provided for removing air and other gases from their naturalcollection point in the lower area of the vessel interior in order todecrease the differential temperature between the steam and the liquidto be sterilized. The flow pattern of the liquid is such that physicalcontact with any surface within the vessel which is hotter or equal intemperature to the product is avoided and the product itself serves asan insulating barrier between the stream and components of the vessel.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view of an infusion heating apparatus accordingto the invention;

FIG. 2 is a cross sectional view of the upper portion of the infusionheating apparatus of FIG. 1;

FIG. 3 is an enlarged cross sectional view of the upper portion of theinfusion heater of FIG. 1;

FIG. 4 is a cross-sectional view of the lower portion of the infusionheating apparatus of FIG. 1;

FIG. 5 is a view taken along line 5--5 of FIG. 2;

FIG. 6 is an enlarged cross-sectional area view of the level controldevice in the lower portion of the infusion heater of FIG. 1;

FIG. 7 is a schematic diagram of the infusion heating system accordingto the invention;

FIG. 8 is a diagrammatic view of the spray and film pattern in the upperportion of the infusion heater according to FIG. 1;

FIG. 9 is a cross-sectional view similar to FIG. 5 with an alternateorientation of spray nozzles;

FIG. 10 is a cross-sectional view of the upper portion of an alternateembodiment of a steam infusion heater; and,

FIG. 11 is a cross-sectional view of the lower portion of the alternateembodiment shown in FIG. 8.

DESCRIPTION OF A PREFERRED EMBODIMENT

As is seen generally in FIGS. 1 through 4, the infusion heatingapparatus includes a vertically upstanding hollow closed vessel 10having a generally cylindrical central portion 11 and frusto-conicalshaped upper and lower portions 12 and 13 respectively. The top of thevessel 10 has a cover plate 14 which seals the upper portion 12 andincludes an upstanding central cylindrical collar 15. A second or lowercollar 16 is provided with a gasket 17 for sealing the joint betweencover 14 and a film forming plate 18. A central cylindrical portion 20extends downwardly into the interior of the vessel 10 through thecollars 15 and 16, gasket 17 and plate 18. Concentrically spaced withinthe cylindrical portion 20 is a steam delivery pipe 21 forming aninsulating air space 22 between cylinder 20 and the steam pipe 21. Thelower end of the cylinder portion 20 has a gradually diminishingfrusto-conical portion 23 which terminates in a welded sealed joint 24with the steam pipe 21.

The steam delivery pipe 21 extends through the cylindrical portion 20downwardly to an internally threaded portion 25 to which is attached asteam distribution cylinder 26 consisting of porous sintered stainlesssteel. The lower end of the cylinder 26 is sealed by an end cap 27.

Also extending concentrically within the steam delivery pipe 21 is ahollow tube 30 which extends downwardly through the entire length of thedelivery pipe 21 and steam distribution cylinder 26, through the end cap27 and downwardly within the vessel 10 to form a projecting portion 31terminating within the lower interior portion of the vessel 13.

As shown in FIGS. 2, 3 and 5, the upper interior portion 12 of thevessel 10, has an annular space 35 defined by the side walls of theupper portion 12, the second cylindrical collar 16, the cover 14 and thefilm forming plate 18 which extends around the cylindrical portion 20and is spaced downward from the cover plate 14. The outer diameter ofthe film forming plate 18 is spaced from the side wall of the uppervessel portion 12, and has a frusto-conical teflon coated disc 37attached to its lower outer edge and shaped so that the sides of thedisc 37 extend in close spaced proximity to the interior side wall ofthe upper vessel portion 12 to form a continuous passageway 38 betweenannular space 35 and the vessel 10 interior and extending entirelyaround the upper interior surface of the vessel portion 12. The spacedrelationship may be maintained by a plurality of small protuberantmembers 39 formed around the sides of the disc 37 and extending betweenthe disc 37 and the interior surface of the upper vessel portion 12 asseen in FIG. 3. As is also seen in FIGS. 2, 3 and 5, a plurality ofspray nozzles 40 are mounted on the bottom of the plate 18 adjacent toand surrounding tube 20 and have communicating flow passages 41 betweenthe annular space 35 and the nozzles 40. Spaced beneath the nozzles 40and extending outwardly and downwardly from the central tube 20 is afrusto-conical shaped deflector baffle 42 whose purpose and functionwill be explained hereinafter. Also extending through the cover 14 is aproduct inlet aperture 45 to which a supply pipe 46 is attached forintroducing liquid product into the annular space 35 as shown in FIGS. 2and 3.

As seen in FIGS. 2 and 3, the steam supply pipe 21 has a steam inletpipe 48 for connection to a source of steam in a conventional manner,the details of which are not shown or necessary. The upper end of thepipe 21 outside the cylinder is capped by an end plate 49 through whichthe air pipe 30 may pass through the use of a conventional pipe fitting50.

Referring to FIGS. 1 and 4, the lower end 13 of the vessel 10 terminatesin a flange 51 to which a second frusto-conical flanged member 52 isattached by conventional means such as bolts 53 and nuts 54 to form ajoint sealed by a gasket 55. The lower end of the second frusto-conicalmember 52 terminates in a cylindrical outlet portion 56 containing alevel control apparatus 57 shown in greater detail in FIG. 6. The levelcontrol apparatus 57 comprises a hollow spherical member 58 having anattached hollow cylindrical tube 59 extending through an aperture 60formed in the side of the outlet portion 56. A cylindrical outlet pipe61 extends around the aperture 60 terminating in a flange 62 to which anend cap 63 is attached in a sealed relationship by means of a clamp 64and gasket 65 at the joint. Tube 59 extends through the end cap 63 andis secured by means of a nut 66 and held in a sealed relationship withthe end cap by means of an O-ring 67. Also, centrally located within thetubular member 59 is an activating rod 68 which is attached at one endto the spherical member 58 and extends through the tube 59 to aterminating point 70 having a downwardly extending plunger 71 attachedthereto with the plunger 71 activating a pneumatic transmitter valve 72whose details are not shown. The spherical member 58 will thus floatwithin a level of liquid at the outlet portion 56 and move upwardly ordownwardly with the level. As member 58 moves up or down, it willdeflect the activating rod 68 thereby moving the plunger 71 to activatethe transmitter valve 72. Details of the transmitter valve are not apart of this invention, but those skilled in the art will appreciatethat such valves will typically generate a signal which may bepneumatic, electrical, hydraulic or even mechanical, in response tomovement of the plunger 71. The generated signal basically senses levelchanges, whether positive or negative, and is used to control otherdevices in response to the level changes.

As will be appreciated by those skilled in the art, other methods ofcontrolling the level of fluid product at the outlet of the vessel havebeen known and used in infusion heaters. For example, the use of gammarays emitted to a target have been used to detect the level of fluidwithin the outlet with sensing devices then being used to adjust theflow controls and rates. Additionally, pressure sensitive diaphragms forsensing differential pressure within the outlet based upon the level ofliquid have also been employed to provide signals to flow controldevices. These are not described in detail as their features andprinciples will be appreciated by those skilled in the field.

A schematic depiction of the system is shown in FIG. 7, where the vessel10 receives saturated steam from a source 80 via the steam inlet 48which flows into the steam line 21 and is gently diffused into theinterior of the vessel 10 through the porous distribution cylinder 26 asindicated by arrows 81. Liquid product to be treated from a conventionalsource 84 is directed by pump 85 through a flow control valve 86 toinlet pipe 46 and is introduced into the vessel 10 as will be describedin more detail later. The product is heated by steam infusion as itflows by gravity to the bottom of the vessel 10 where it is collected inthe product removal cylinder 56 and removed via an outlet flow line 87by a pump 88. Signals from the level sensing apparatus 57 control therate of introduction of product by controlling the flow control valve 86via the transmitter 72. Air and gases released from the product andcondensing steam within the vessel 10, being heavier than steam, flow tothe cone bottom 13 of the vessel 10 and are removed by the air outletline 31 and flow upward through a liquid trap valve 90 which permitonly, gases to flow. A pressure gauge 92 is provided for monitoring thepressure of the gases being removed which flow through check valve 93. Amanual control valve 94 and flow meter 95 is provided for monitoring andcontrolling the rate of discharge of air and gases to the atmosphere toavoid discharging of steam from the vessel 10.

Having thus described the basic apparatus, the method of treatingproduct according to the invention will now be described. Since theinvention is primarily directed to sterilization of milk products suchas ice cream mix, the preferred method will be described with referenceto those types of product and with further reference to FIGS. 2 and 3.The liquid mixture is preheated to a temperature of 180° to 185° F. (82°C. to 85° C.) and introduced into the annular chamber 35 via the productsupply pipe 46. The product flows from the annular chamber 35 throughthe passageway 38 and is simultaneously sprayed into vessel 10 by spraynozzles 40. A flowing film of product adhering to the interior sides ofthe upper vessel 12 forms due to surface tension, the orientation ofpassageway 38, velocity of flow, and the action of the spray. This formsan initial film 100 of flowing product shown in FIGS. 3 and 8 while atthe same time, additional product is sprayed from the nozzles 40outwardly and downwardly into contact with the film 100 and also intothe upper portion of the vessel interior.

The product spray pattern is best illustrated in FIGS. 5 and 8. Theplurality of nozzles 40 arranged around and adjacent to the cylinder 20spray the product as indicated by arrows 102 outwardly in overlappingfinger-like patterns depicted in FIG. 5 as well as downwardly as shownin FIG. 8. The pattern of the spray is extremely concentrated andintense and effectively forms a generally annular area of concentratedspray depicted by phantom line 103 in FIG. 8. This annual space 103 isin the uppermost portions of the vessel 10 and serves as an effectiveinsulating barrier between steam introduced into the vessel below andthe top of the vessel 10. This barrier effectively precludes the steamfrom contacting the upper interior surfaces of the vessel 10 therebypreventing burn-on of the product. The concentrated spray pattern iscontinually renewed as the droplets of spray fall by momentum andgravity through the interior of the vessel 10 down to the bottom portion13 or combine with the already flowing film 100 along the sides of thevessel interior walls. The deflector 42 acts to divert the sprayoutwardly and away from the steam distribution cylinder 26 as indicatedby arrow 104 to again avoid burn-on in any of the central portions ofthe vessel. The flowing product film 100 is initially relatively cool asit flows from the annular space 35 and is gradually heated as it flowsdown the vessel side walls. The film 100 thus effectively acts as aninsulating barrier between the steam and the vessel side walls toprevent the side walls themselves from ever reaching a temperature equalto or in excess of the film itself.

During the process, saturated steam at a temperature of approximately293° F. (145° C.) is introduced via steam line 21 and diffused gentlyinto the interior of the vessel 10 through the porous distributioncylinder 26. An operating pressure of 45 to 50 pounds per square inch isused, and the diffused steam heats the falling spray droplets and filmby infusion, rapidly heating the product to the sterilizationtemperature. As the steam is absorbed by the product and the producttemperature increases, the heavier air and gases released from theproduct itself and remaining after the steam is condensed will gravitateto the lower end of the vessel 10. The air removal pipe extension 31extends within the lower portion 13 of the vessel 10 and provides anoutlet for the air and gases at the optimal location. By providing forthe removal of such air and gases, the operating temperature andpressure of the steam can be more closely controlled and maintained inproximity to the desired sterilization temperature of the product.

An alternate orientation of spray nozzles 40 is shown in FIG. 9 wherethe spray pattern indicated by arrows 105 from each nozzle ranges fromgenerally radial to tangential in direction relative to the cylinder 20.The nozzles 40 are also angled downwardly as well. This orientationprovides not only overlapping of the spray patterns but would alsocreates a generally circular swirling movement of spray within thecooler annular space 103. With this orientation of the nozzles 40, theliquid flowing through passageway 38 may also be directed to emerge in aswirling pattern as well. This could be accomplished through the use ofgrooves, directional fins or vanes or other devices to impart a swirl tothe liquid film in the same general direction of rotation as the spraypattern shown by arrows 105 in FIG. 9.

It will be apparent to those skilled in the field that the method andapparatus thus described is extremely simple, avoids the problem ofproduct burn-on by utilizing the product itself to form an insulatingbarrier to prevent steam from heating any surfaces contacted by theproduct itself. This minimizes undesirable changes in flavor of the milkproduct. Furthermore, by providing an air removal system for removinggases and air from the vessel, a lower temperature difference betweenthe steam itself and the product results which also minimizes flavorchanges. The air space 22 between the steam inlet line 21 and thecylindrical portion 20 extending downward from the cover 14 serves as aninsulating barrier to prevent heating of the product within the annularspace 35, and the initial flowing film layer 100 and spray patterncombined with deflector 42 minimizes the possibility of the productcontacting any exposed hot surface within the vessel 10.

The physical characteristics of the spray nozzles and the dimensions ofthe passageway 38 will vary depending upon the type of fluid product andits viscosity and surface tension. In the case of an ice cream mix, forexample, passageway 38 has a dimension of approximately 0.055" (0.14cm.) and the spray nozzles are configured to release spray droplets ofapproximately 0.0625" (0.16 cm.) diameter. The shower of droplets inaddition to falling directly downward to the bottom of the vessel 10will also, in part, combine with the flowing film 100 along the sides ofthe vessel. Due to the high efficiency of heating, ice cream mix, forexample, may be processed at the rate of 3,500 gallons per hour with thefilm introduced at velocities of two to three feet per second ofdownward flow. The apparatus may also be adjusted to eliminate theflowing film forming passageway 38 and simply fashion the spray nozzles40 to create the cooler spray barrier at the top of the vessel and alsodirect spray at a sufficient rate to form the flowing film 100 along theinterior vessel walls by use of the spray nozzles 40 alone. These samebasic principles will apply as far as the heating and the insulatingcharacteristics of the product to prevent product contact with surfaceshotter than or equal to the product temperature itself. Anotheralternative is to eliminate the spray nozzles and simply introduce filmthrough the passageway 38 and heat the flowing film alone. The coolerproduct flowing from the annular chamber 35 will keep the vessel sidewall surface cooler than the steam itself and also serve to cool themember 37 and again forms a continuous insulating barrier along theinterior side walls of the vessel. In this alternative, the steam simplyheats the flowing film with no free falling spray droplets.

An alternate embodiment of the invention is shown in FIGS. 10 and 11which contains certain features common to the vessel of FIG. 1. To theextent that similar components are shown, they are referenced with thesame reference numbers used in connection with FIG. 1 with the suffix"a" added. The vessel 10a comprises a cylindrical upper portion 11a andfrusto-conical lower portion 13a extending down to the collection point56a, however, the upper portion of the vessel 10a remains cylindricaland terminates in a flanged upper portion 105 to which the cover 14a isaffixed by conventional means such as bolts 106 and nuts 107 with agasket seal 108 at their joint. Additionally the lower central cylinder16a has two gasket seals 17a between spacer 16a and the cover 14a andalso between the spacer 16a and the film forming plate 18a. All theremaining components are arranged similarly to those shown in FIG. 1 anddetailed description is unnecessary.

The alternate embodiment shown in FIGS. 10 and 11, permits the film ofproduct to formed and introduced solely through the use of the filmforming plate 18a and disc member 37a and makes the use of the spraynozzles 40a optional. If no spray is used through the nozzles 40a, thefilm simply flows by gravity and down to the interior side walls due tosurface tension, and the direction and velocity of the introduced film.In addition the rate of liquid flow within the annular space 35a and outthe passageway 38a serves as a cooling medium. With this alternativedesign, the spray nozzles may also be used to add product to the flowingfilm as well as the interior space of the vessel 10 in the same manneras previously described.

Because of the design of the apparatus, it is extremely easily cleanedas the configuration of the vessel and product inlet can in effectbecome a clean-in-place system. To clean the vessel, it is merelynecessary to inject cleaning fluid solutions through the product inletline 46 into the annular space 35 thereby permitting the cleaningsolution to flow through the nozzles 40 and space 38 covering the entireinterior of the vessel with cleaning solution.

While the invention has thus been described in relation to a method andapparatus for treating milk, others skilled in the art will appreciatethat other food products in liquid form may also be sterilized as wellsuch as flavored milk, half and half, dairy creams, whipping creams,condensed milk, ice cream milk, shake mix, puddings, custard, fruitjuices, etc. Adjustments to the operating temperature and flow rates maybe necessary but these variations will be recognized and easilyaddressed by those skilled in the field.

Accordingly, while one preferred embodiment of the invention has thusbeen described, the scope of the invention is not to be limited therebybut solely based upon an interpretation of the claims which follow.

I claim:
 1. A method of treating liquid food product by steam infusion comprising the steps of:a. introducing liquid food product to be treated at the top of a pressure vessel to form a continuously flowing film of product down the vessel side walls; b. disbursing saturated steam within said vessel at a point centrally within said vessel and intermediate its top and bottom portions to heat said liquid product; c. removing air and gases released from said liquid product and said steam from the lower portion of said vessel; and, d. removing heated liquid product from the bottom of the vessel.
 2. A method as set forth in claim 1 wherein said liquid product is introduced through an annular passageway contiguous to the upper side walls of said vessel.
 3. A method as set forth in claim 2 wherein said liquid product is also introduced through a plurality of spray nozzles at the upper portion of said vessel to form an insulating barrier of spray interposed between said vessel top and the level of steam disbursing.
 4. A method as set forth in claim 3 including the step of deflecting and directing product spray away from said center of said pressure vessel.
 5. A method as set forth in claim 1 wherein said liquid product is introduced through a plurality of spray nozzles at the upper portion of said vessel to form an insulating barrier of spray interposed between said vessel top and the level of steam disbursing.
 6. A method as set forth in claim 5 including the step of deflecting and directing product spray away from said center of said pressure vessel.
 7. A method of treating a liquid food product by steam infusion comprising the steps of:(a) introducing liquid food product to be treated in the upper portion of a pressure vessel to form a continuously flowing film of product down the vessel side walls, (b) disbursing saturated steam within said vessel at a point centrally within said vessel and intermediate the upper and lower portions of said vessel to heat said liquid product, (c) removing heated liquid product from the lower portion of the vessel, (d) forming an insulating barrier of spray by also introducing liquid food product through a plurality of spray nozzles interposed between said upper portion of said vessel and the level of steam disbursing.
 8. The method according to claim 7 wherein a portion of said liquid product is introduced through an annular passageway continguous to the upper side walls of said vessel.
 9. The method according to claim 7 including the step of deflecting product spray away from the center of said pressure vessel.
 10. The method according to claim 7 including the step of removing air and gases released from said liquid product and said steam from the lower portion of said vessel.
 11. The method according to claim 8 including the step of deflecting product spray away from the center of said pressure vessel.
 12. The method according to claim 11 including the step of removing air and gases released from said liquid product and said steam from the lower portion of said vessel. 